Ok, I feel bad now, it's the second time I was wrong in this post.
This is a bug, but I swear it wasn't happening yesterday! Your project must have infected my NWJS with this bug :)
I reported it on Scirra bug tracker:
github.com/Scirra/Construct-3-bugs/issues/3465
.
To work around this bug - after opening the dialog you can deactivate the group with "On LoadButton clicked" event, and when dialog is closed/cancelled, wait 0.5 seconds before re-activating the group.